An image scanner or other image reading apparatus typically has a composition along the lines of that shown in FIG. 19. In FIG. 19, an image sensor unit 1 converts optical information from an original 7, placed on a original glass plate 4, into an electrical signal. A movement unit 2 alters the relative positions of the original 7 and the image sensor unit 1, and alters the reading position of the original 7. A control unit 3 drives the image sensor unit 1 and the movement unit 2, and processes the electrical signal from the image sensor unit 1.
When inputting a color image, the image scanner or other image reading apparatus performs a color separation of the color image. Conventionally, the typical manner of performing the color separation has been to apply a color filter with differing spectral transmittance for each of red, green, and blue, or RGB, as a light sensing element, which receives light reflected from the original, illuminated by a fluorescent lamp or other white light source. In recent years, however, an image reading system with a light source switching method has become common, wherein color separation is performed by the light source by using three light emitting diodes(LEDs) with differing RGB emission spectrum as a light source, and driving the respective RGB LEDs in time sequence.
FIG. 8 shows a cross-sectional figure of a contact image sensor (CIS), which is the image sensor unit 1 in the prior art of the light source interchange scheme.
In FIG. 8, light being radiated from a light source 41, which is positioned on an end of a light guide 12, is entered within the light guide 12 and guided in a moderately long direction, and substantially homogeneously irradiates the reading position of the original 7, in a line shape along the main scanning direction of the original 7 that is placed on the original glass plate 4. A reflected light from the original irradiated in the above described manner is concentrated by a lens array 14 and transferred to a sensor array 15 provided on a sensor substrate 16 to be converted into an electrical signal. The electrical signal corresponding to the original is output via a connector 17. In FIG. 8, reference numeral 13 denotes a frame for fixing component members such as the light guide 12, the lens array 14 and the sensor substrate 16 and the like to respective predetermined positions.
The light source 41 comprises three LEDs of light each having different emitting wavelengths from each other: 41r, 41g, and 41b, which radiate red, green, and blue light, respectively. As shown in FIG. 9, the LEDs are wired using a common wire and an individual wire for each LED, and the LEDs 41r, 41g, and 41b are constituted such that they can individually control the lighting.
The image sensor unit 1 and the movement unit 2, which alters the relative positions of the original 7 and the image sensor unit 1, are driven according to prescribed timing by the control unit 3, which converts the optical information from the original 7 into an electrical signal. The operation of the control unit 3 is described in the following section.
FIG. 10 is a block diagram depicting conventional technology, and a composition of an image reading apparatus.
During an image reading operation, the control unit 3 drives the image sensor unit 1, using a light source controller 33 and a sensor controller 34, in accordance with a timing chart depicted in FIG. 11. A sensor array 15, shown in FIG. 8, which constitutes the image sensor unit 1, treats one cycle of the synchronizing signal SP, inputted from an external device, as an image information accumulation cycle TS, in which it integrates image information during one operating cycle, outputs the integrated image information in the next operating cycle.
A cycle TC, comprising three image information accumulation cycle TS of the sensor array 15, is treated as one cycle of color reading operation, and the light source 41 LEDs 41r, 41g, and 41b are turned on, individually and in sequence, by control signals φLr, φLg, and φLb, using the light source controller 33 as depicted in FIG. 12, in every operating cycle during the cycle TC. Consequently, the light source switching method is performed, wherein the colors of the original 7 are separated by using the LEDs 41r, 41g, and 41b emission spectrum and the image information output or line output, OS(r), OS(g), and OS(b), which has been color separated in line sequence, may thus be obtained.
The control unit 3 drives the image sensor unit 1, a movement unit controller 32 drives the movement unit 2 synchronously therewith, changing the relative positions of the portion of the original 7 that is to be read and the image sensor unit 1, and collecting two-dimensional image information for the original 7. Following is a description of conventional image signal processing as performed by the control unit 3.
The control unit 3 performs a preparation operation such as the following, prior to a reading operation of the original 7.
Controlling the movement unit 2, the control unit 3 moves the image sensor unit 1 to a prescribed initial position. In FIG. 19, a white reference plate 5 is positioned upon the original glass plate 4 as pertains to the initial position, and the control unit 3 adjusts the individual light intensity emitted by the respective LEDs 41r, 41g, and 41b that constitute the light source of the image sensor unit 1.
The reason for adjusting the individual light intensity emitted from the respective LEDs 41r, 41g, and 41b is to correct for a fluctuation in the luminous efficiency and sensor sensitivity on the part of the respective LEDs 41r, 41g, and 41b, to meet an output level of an RGB line output OS(r), OS(g), and OS(b) into conformity with an input range VH of an A/D converter 35, and obtain image information with an optimal S/N.
An example of a technique that adjusts the light intensity emitted from the respective LEDs 41r, 41g, and 41b would be a method so used that adjusts either light intensity or light cycle of the respective LEDs 41r, 41g, and 41b, as pertains to the image information accumulation cycle TS. With regard to the light source controller 33 depicted in FIG. 12, a lighting condition register 33a is configured for setting a prescribed light cycle TD, and the white reference plate 5, which is positioned upon the original glass plate 4, is read. The maximum values of the line output for the respective colors, as pertains to the sensor array 15, are assigned to Vr1, Vg1, and Vb1, respectively, and values Tlr, Tlg, and Tlb for the respective LEDs 41r, 41g, and 41b that are set in the lighting condition register 33a, as the VH maximum values of the target output, are configured as per the following respective equations 1 through 3:TLr=TD×VH/Vr1  equation (1)TLg=TD×VH/Vg1  equation (2)TLb=TD×VH/Vb   equation (3)
As a result, the light intensities emitted by the LEDs 41r, 41g, and 41b are respectively adjusted by light cycles, and output levels of the RGB line outputs OS(r), OS(g), and OS(b) are equivalent thereto, conforming with the input range VH of the A/D converter 35.
Reference data is obtained and stored in a memory 37 that is used by a shading corrector 36 that compensates for an offset error or a gain error contained in an output signal from the sensor array 15. Described specifically, the light source 41 is put into a lights out state, and offset correction data D is obtained and stored in the memory 37. The light cycles of the three LEDs 41r, 41g, and 41b that constitute the light source 41 are respectively set to the light cycles Tlr, Tlg, and Tlb, corresponding to their corrected light intensities as described above. The three LEDs 41r, 41g, and 41b, with light cycles Tlr, Tlg, and Tlb, are respectively turned on in sequence, and gain correction data Wr, Wg, and Wb are obtained and stored in the memory 37.
After performing the preparation operation, the control unit 3 drives the image sensor unit 1 and the movement unit 2, respectively, and converts optical information of the original 7 into an electrical signal. An analog signal that is outputted by the image sensor unit 1 is converted to digital data by the A/D converter 35.
The shading corrector 36 performs, for example, computations in the following equations 4 through 6 for the respective color line output Sr, Sg, and Sb of the sensor array 15, thus correction for the offset error or the gain error, and obtaining a normalized line output [R], [G], and [B]:[R]=(Sr−D)/(Wr−D)  equation (4)[G]=(Sg−D)/(Wg−D)  equation (5)[B]=(Sb−D)/(Wb−D)  equation (6)
The normalized line output [R], [G], and [B], having had shading correction for, is temporarily stored in a line memory 38. Time delay of image signal that is color separated in line sequence is adjusted. RGB normalized output signals Ri, Gi, and Bi, that correspond to a given position on the original 7, are extracted in sequence, and color space conversion processing on the RGB normalized output signals is performed by a color space converter 39. FIG. 13 depicts a view explaining a color space conversion using the color space converter 39. In FIG. 13, a 3×3 correction matrix M is used to perform the color space conversion processing. RGB normalized output signals Xi, Yi, and Zi are thus outputted, having had the color space conversion processing performed thereon.
Image data that has been subjected to colorimetry by an image input device (input device) is reproduced by an image output device (output device), such as a display, a printer and the like. The image input device or the image output device, however, possesses its own unique color space due to a light source, a filter, or a pigment and the like. Hence, in order to obtain a desired color reproduction while connecting the image input device to the image output device, it is necessary a signal processing taking the difference between their color spaces into account. Although a single color space converter for converting the color space of the image input device into the color space of the image output device will be sufficient if the respective color spaces have a fixed one-to-one correspondence, the inputted image data will have a variety of applications, and thus, the output device cannot be specified, as a rule.
As a consequence, the image input device converts image data having a unique color space of an image input device into common standard device-independent color space, and outputs the converted result. Upon reproducing the image data by an image output device, it is constructed such that the image data having the standard color space is converted to the color space of the image output device. Color spaces as CIEXYZ or CIELAB are used as the standard color space.
Coefficients of the 3×3 correction matrix M that is contained within the color space converter 39 are calculated, for example, by a sensor signal vis-à-vis N color charts for which tri-stimulus values are established. More concretely, assigning the tri-stimulus value matrix of the target N colors to TN, and the sensor output matrix vis-à-vis the target N colors to UN, allows deriving the correction matrix M by the following equation 7, as the mean square error of TN and T′N, i.e., MU, goes to its minimum:M=[TNUNt][UNUNt]−1  equation (7)
The ISO/DIS 12641 IT8 color chart is recognized as a standard color chart that is used in such color proofing.    Patent Related Literature 1: Japanese Patent Laid-Open No. 61-148959 (KOUKAI SHOWA 61-148959);    Patent Related Literature 21: Japanese Patent Laid-Open No. 08-275006 (KOUKAI HEI 08-275006);    Patent Related Literature 3: Japanese Patent Laid-Open No. 11-243492 (KOUKAI HEI 11-243492)